Willard Libby developed radiocarbon dating as a method to measure radioactivity. Carbon-14 is a weakly radioactive isotope of Carbon; also known as radiocarbon, it is an isotopic chronometer. Radiocarbon dating is only applicable to organic and some inorganic materials (not applicable to metals).
Carbon dating is a variety of radioactive dating which is applicable only to matter which was once living and presumed to be in equilibrium with the atmosphere, taking in carbon dioxide from the air for photosynthesis.
Radiocarbon dating or in general radioisotopic dating method is used for estimating the age of old archaeological samples. For example, age of the earth, moon, rocks, and mineral deposits can be determined by using the principle of radioisotopic dating.
This technique was developed by Williard Libby. He was awarded Nobel Prize for this work.
In the upper atmosphere, nitrogen (147N) is bombarded by cosmic ray to produce 146C : (146C is a radioactive isotope of carbon).
147N + 10n → 146C + 11H
Radioactive carbon-14(146C) gets converted to radioactive carbon dioxide (14CO2). This radioactive 14CO2 is taken up by plants during photosynthesis. 146C is radioactive and decays by β-emission.
146C → 147N + 0-1e (β-particles)
Since, 146C is being continuously formed and consumed (due to β-emission decay), hence an equilibrium concentration of 146C is maintained in all the living plants. However, when a plant dies, it can no longer fix up radioactive 14CO2. As a result, the concentration of 146C in it starts decreasing. The half-life of a 146C is 5760 years. Thus, in 5760 years, the concentration of 146C is lowered to half (50%) of its initial concentration, and after another 5760 years, its concentration gets lowered to 25 %( 50% of the 50%) of the initial concentration. Thus in 11,520 years the 146C concentration is reduced to one fourth of its initial concentration. Thus, by measuring the concentration of 146C in a dead carbon-containing object, and knowing the concentration of 146C in a living plant, were can estimate the age of the object (the age of the object means the number of years ago when plant should have died), by using the formula.
Age of the carbon containing object
= t1/2/log ×2 log (Concentration of 146C in a living plant)/(concentration of 146C in the given dead object)
Where, t1/2 for 146C is 5760 years.
Estimating the age of a carbon-containing object by measuring the concentration (or activity) of 146C in it, is called radiocarbon dating.